Horton

Re: [HACKERS] Custom compression methods

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>, Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>
Cc: Евгений Шишкин <itparanoia@gmail.com>, Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Oleg Bartunov <obartunov@gmail.com>, Craig Ringer <craig@2ndquadrant.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2017-12-01T20:47:43Z
Lists: pgsql-hackers 

On 12/01/2017 08:48 PM, Alvaro Herrera wrote:
> Ildus Kurbangaliev wrote:
> 
>> If the table is big, decompression could take an eternity. That's why i
>> decided to only to disable it and the data could be decompressed using
>> compression options.
>>
>> My idea was to keep compression options forever, since there will not
>> be much of them in one database. Still that requires that extension is
>> not removed.
>>
>> I will try to find a way how to recompress data first in case it moves
>> to another table.
> 
> I think what you should do is add a dependency between a column that
> compresses using a method, and that method.  So the method cannot be
> dropped and leave compressed data behind.  Since the method is part of
> the extension, the extension cannot be dropped either.  If you ALTER
> the column so that it uses another compression method, then the table is
> rewritten and the dependency is removed; once you do that for all the
> columns that use the compression method, the compression method can be
> dropped.
> 

+1 to do the rewrite, just like for other similar ALTER TABLE commands

>
> Maybe our dependency code needs to be extended in order to support this.
> I think the current logic would drop the column if you were to do "DROP
> COMPRESSION .. CASCADE", but I'm not sure we'd see that as a feature.
> I'd rather have DROP COMPRESSION always fail instead until no columns
> use it.  Let's hear other's opinions on this bit though.
> 

Why should this behave differently compared to data types? Seems quite
against POLA, if you ask me ...

If you want to remove the compression, you can do the SET NOT COMPRESSED
(or whatever syntax we end up using), and then DROP COMPRESSION METHOD.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Commits

  1. docs: Update TOAST storage docs for configurable compression.

  2. Further tweaking of pg_dump's handling of default_toast_compression.

  3. Fix interaction of TOAST compression with expression indexes.

  4. Tidy up more loose ends related to configurable TOAST compression.

  5. Short-circuit slice requests that are for more than the object's size.

  6. Mostly-cosmetic adjustments of TOAST-related macros.

  7. Remove useless configure probe for <lz4/lz4.h>.

  8. Error on invalid TOAST compression in CREATE or ALTER TABLE.

  9. docs: Fix omissions related to configurable TOAST compression.

  10. More code cleanup for configurable TOAST compression.

  11. Bring configure support for LZ4 up to snuff.

  12. Make compression.sql regression test independent of default.

  13. Use valid compression method in brin_form_tuple

  14. Fix up pg_dump's handling of per-attribute compression options.

  15. Allow configurable LZ4 TOAST compression.

  16. Fix inconsistencies in the code

  17. Mostly-cosmetic improvements in memory chunk header alignment coding.

  18. Allow numeric to use a more compact, 2-byte header in many cases.